Megabacteria are microscopic organisms which are forty times larger than most bacteria. They have at various times been called bacteria, fungi and lactobacilli, but are now deemed to be in a microscopic class of their own.

Megabacteria's appearance in Australia was first reported in the early 1980's, and it has been the subject of increasing studies since then, most recently at the University of Queensland.

Megabacteria typically cause a weight loss syndrome with bulky soft, dark faeces. Food is poorly digested as the organism infects the birds gastrointestinal tract, and as a result, the birds appetite is usually increased. The course of the disease is usually more than one month, although sudden death after acute vomiting episodes does occur.

The transmission of Megabacteria is not yet certain, but the main theory is that infected faeces are ingested by other birds and so passed on. Whether egg transmission or beak to beak spread occurs is being studied.

The limits of the incubation period of Megabacteria in birds is not known fully. There has been a case of one bird (obtained at 7 weeks of age) developing signs 4 years later. It is likely that birds can be carriers with no symptoms for long periods, and that megabacteria require a certain set of circumstances to cause disease e.g. nutritional deficiency/stress, or presence of another bacterial infection. There are certainly many instances where megabacteria are seen in faeces of dinically healthy birds, just as there are cases where wasted, thin birds have no mega bacteria in their droppings but large numbers of them in their stomach when examined at post mortem.

The treatment and control of megabacteria is currently the area of most research. Most antibiotics and antifungal drugs have been trialled, with very few showing consistent, effective results. However, testing and treating positive or suspect birds with some anti-fungal treatments is the most successful approach, along with good hygiene and quarantine practices.